1. Field of the Invention
The present invention relates in general to a hybrid drive system for driving an automotive vehicle, and more particularly to such a hybrid drive system such that a vehicle drive force is substantially constant under the same vehicle running condition, even in different modes of operation of the hybrid drive system.
2. Discussion of the Related Art
For driving an automotive vehicle, there is known a hybrid drive system including (a) an engine operated by combustion of a fuel, and (b) an electric motor operated with an electric energy, and having a motor-aided engine drive mode in which the engine is used as a primary drive power source while the electric motor is used as an auxiliary drive power source. An example of this type of hybrid drive system is disclosed in JP-A-3-121928. This hybrid drive system has a normal engine drive mode in which only the engine is operated to drive the vehicle, and a motor-aided engine drive mode in which the electric motor is operated together with the engine to drive the vehicle, so that the required drive force or drivability is obtained while minimizing fuel consumption and exhaust gas emissions. The normal engine drive mode is usually or normally selected, and the motor-aided engine drive mode is selected when the vehicle is running under a high load condition, that is, when the load acting on the engine (as represented by an amount of operation of the accelerator pedal, for example) is higher than a predetermined upper limit.
If the electric motor is used as the auxiliary drive power source under any running condition of the vehicle as long as the engine load is higher than the predetermined upper limit, the overall vehicle drive force corresponding to a given amount of operation of the accelerator pedal undesirably varies, unexpectedly to the vehicle operator, depending upon whether the electric motor is inoperable due to excessive reduction of the electric energy amount stored in an electric energy storage device, more precisely, due to reduction of the stored electric energy amount below a predetermined lower limit.